Rubber goods such as tire treads often are made from elastomeric compositions that contain one or more reinforcing materials such as, for example, particulate carbon black and silica; see, e.g., The Vanderbilt Rubber Handbook, 13th ed. (1990), pp. 603-04.
Good traction and resistance to abrasion are primary considerations for tire treads; however, motor vehicle fuel efficiency concerns argue for a minimization in their rolling resistance, which correlates with a reduction in hysteresis and heat build-up during operation of the tire. These considerations are, to a great extent, competing and somewhat contradictory: treads made from compositions designed to provide good road traction, particularly in wet conditions, usually exhibit increased rolling resistance and vice versa.
Filler(s), polymer(s), and additives typically are chosen so as to provide an acceptable compromise or balance of these properties. Ensuring that reinforcing filler(s) are well dispersed throughout the elastomeric material(s) both enhances processability and acts to improve physical properties. Dispersion of fillers can be improved by increasing their inter-action with the elastomer(s). Examples of efforts of this type include high temperature mixing in the presence of selectively reactive promoters, surface oxidation of compounding materials, surface grafting, and chemically modifying the polymer, typically at a terminus thereof.
For compounds employing silica as a particulate filler, silane coupling agents can increase interactivity between the polymer chains and the silica particles. These types of materials can present some processing difficulties, such as release of ethanol during mixing and/or scorching at high processing temperatures (due to the presence of free sulfur groups in certain types of silanes).
Polymers modified with alkoxysilanes exhibit good interactivity with silica filler, but such compounds can exhibit such high compound Mooney viscosities that they are difficult to process. This tendency can be counteracted by reducing the molecular weight of the polymer, but low molecular weight polymers are difficult to dry and form into bales and also can exhibit cold flow during storage.
This type of inadequate resistance to cold flow is part of another set of competing characteristics: polymers having acceptable cold flow properties often are difficult to process and vice versa. Previous efforts have been directed at blending different types of polymers so as to achieve an acceptable balance of storage and processing performance; see, e.g., U.S. Pat. Nos. 3,278,644, 3,281,389, and 3,242,129.
Elastomers having tin-carbon bonds, often produced through use of tin coupling agents such as SnCl4, typically exhibit good hysteretic performance but marginal wet traction performance. Tin-coupled polymers also exhibit desirable storage characteristics. Examples of relatively recent work in this area can be found in, e.g., U.S. Pat. No. 6,008,295 (combination of tin-coupled and alkoxysilane-terminated polymers) and U.S. Pat. No. 7,279,531 (O-containing coupling agents such as (Cl3Sn)2O and R(Cl3SnO)2 where R is an alkylene group).
Recent regulatory initiatives directed at tin-containing compounds has accelerated the desirability for identifying and/or developing acceptable tin-free options.